The present invention relates to a process for preparing para-aminophenol, more particularly to a process for preparing para-aminophenol from nitrobenzene by utilizing electrochemical synthesizing method without using either surfactant or any special solvent for dissolution or dispersion of nitrobenzene within electrolytic bath in which working electrodes are consisted of multi-rotating disk electrode (hereinafter refered to as multi-RDE) and solid polymer electrolyte (SPE) electrode.
For the conventional general process for preparing para-aminophenol, it can be mentioned a chemical method in which para-nitrophenol or nitrobenzene is hydrogenated in the present of catalyst. However, such chemical method is not recommendable since the reaction yield thereof is relatively low and moreover, the catalysts used therein are expensive noble metals such as platinum, etc.
To develop a new form of process for preparing para-aminophenol capable of overcoming aforementioned disadvantages in reaction yield and manufacturing cost, recent studies are concentrated on electrochemical synthesizing method (hereinafter referred to "electrolytic synthesizing method") and as its typical example, U.S. Pat. No. 3,338,806 can be mentioned.
Above patent relates to a method for producing para-aminophenol by electrochemically reducing nitrobenzene in electrolyte mixed with ethanol and sulfuric acid wherein since there are produced much undesirable reaction products such as aniline or azoxy benzene, not only reaction yield is low as about 62%, but also having problem that ethanol the solvent should be recovered. Further, although there has been known another electrolytic synthesizing method using 1-propanol as solvent instead of said ethanol, the reaction yield in high density nitrobenzene solution is poor as about 67%, and also having problem in that 1-propanol should be recovered in similar way as in case of the ethanol solvent.
On the other hand, according to the U.S. Pat. No. 4,584,070, as an expedient for eliminating the necessity of solvent recover problem in said manufacturing method, a method is disclosed which disperses surfactant (trialkylamine-N-oxide) simultaneously with forming separate third compartment between positive compartment and negative compartment of electrolytic bath whereby decreasing or preventing diffusion of oxygen produced in positive compartment into negative compartment while removing oxygen dissolved within negative compartment whereby suppressing the producing of azoxybenzene, a unnecessary reaction product.
However, according to the above patent, since the surfactant used in relatively large amount is expensive, and separate third compartment has to be provided within the interior of electrolytic bath, furnishing cost of electrolytic bath becomes increased, and since electrolytic voltage is raised, manufacturing cost is generally raised, on the other hand, as mercury among amalgamated metal electrode used for nagative electrode is dissolved into negative electrode solution, and contaminating reaction products and causing mercury pollution of solution, problems of contamination of environment by mercury as well as reduction of reaction yield according to the speeding up of aging of electrode are appeared.
Thus, in producing para-aminophenol from nitrobenzene by utilizing electrolytic synthesis, since said problem that negative electrode serves as very important factor, there is disadvantage that copper electrode, graphite electrode, and bismuth-coated copper electrode which are used as negative electrode other than amalgamated metal electrode also exhibit electrode dissolution phenomenon in case of reaction yield being high, on the contrary, in case of being extremely stable, reaction yield is low. [J. Jayaraman, K. S. Udupa and H. V. K. Udupa, SAEST, 12, 143 (1977)].
On the other hand, in order to raise the reaction yield, as method for utilizing catalyst and not dependent on improvement of electrode material, a method for scheming the increase of reaction yield by catalytic operation by pouring medium of tin ion within solution is known, however, this method is evaluated such that although reaction yield is raised a little, other problem such as necessity of recovering of medium is appeared, and consequently it is undesirable in economical view point. [H. C. Rance and J. M. Coulson, Electrochem. Acta, 14,283 (1969)].
Since nitrobenzene and reduced products of nitrobenzene are oxidized at positive electrode in case of electrolytic synthesis of para-aminophenol, it is essential to separate positive electrode compartment and negative electrode compartment by providing isolating membrane for the purpose of preventing this, and when considering the reaction mechanism of electrolytic synthesizing reaction of nitrobenzens into para-aminophenol, since hydrogen ion participates in the reaction and moreover, sulfuric acidic electrolyte is used, it is desirable to separate positive electrode compartment and negative electrode compartment by providing cation changing isolation membrane between positive electrode compartment and negative electrode compartment.
Meanwhile, even if being separated by cation-exchange isolation membrane, a part of nitrobenzene and reduced products of nitrobenzene become still migrate to positive electrode compartment, and this serves as a cause of reaction yield dropping.
Particularly, electrolytic synthesizing reaction mechanism of para-aminophenol is composed of producing reaction of intermediate, phenylhydroxylamine by electrochemical reducing reaction of nitrobenzene being of first step, and chemical transposing reaction of phenylhydroxyl amine being of intermediate product into para-aminophenol as second step. In such a two-step reaction mechanism, in case when chemical transposing reaction of second step is not carried out rapidly, phenyl hydroxylamine is reduced electrochemically around the electrode and being converted to anilin, a by-product, and in case when any oxygen exists within negative electrode compartment solution, phenyl hydroxylamine is converted to azoxybenzene and producing unnecessary reaction by-product whereby causing decrease of reaction yield.
Therefore, in order to raise the reaction yield in electrolytic synthesis of para-aminophenol, it is required to increase the reaction rate by promoting agitation of solution and also suppression of the migration of reaction material and reaction product into the other compartment.